This invention relates to the recovery of oil from subterranean reservoirs and, more particularly, to new and improved secondary recovery operations utilizing flood water including a dispersion of a hydrophilic carbon oxide for mobility control.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only a minor portion of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. In the supplemental techniques, which are commonly referred to as secondary recovery operations although in fact they may be primary or tertiary in sequence of employment, fluid is introduced into the reservoir in order to displace the oil therein to a suitable production system through which the oil may be withdrawn to the surface of the earth. The displacing medium may be a gas, an aqueous liquid, such as fresh water or brine, an oil-miscible liquid such as butane, or a water and oil-miscible liquid such as alcohol. Generally, the most promising of the secondary recovery techniques involves the injection into the reservoir of an aqueous flooding medium, either alone or in combination with other fluids.
One difficulty which often is encountered in secondary recovery operations is the relatively poor sweep efficiency of the injected displacing liquid. That is, the displacing liquid exhibits a tendency to channel through certain portions of the reservoir and to bypass other portions. Such poor sweep efficiency is occasioned by differences between the viscosity of the injected displacing medium and the in situ reservoir oil and also by permeability variations within the reservoir. The reservoir may comprise a plurality of fairly well-defined zones of widely diverse permeabilities. The injected displacing fluid preferentially flows through the more permeable zones of the reservoir thus leading to premature breakthrough of the displacing fluid at the production well or wells.
Even when the reservoir exhibits a relatively uniform permeability throughout, a situation referred to as instability fingering may develop in those instances where the viscosity of the injected displacing fluid is significantly less than the viscosity of the in situ reservoir oil. In this situation, the less viscous displacing fluid tends to develop fingers or bulges which may be caused by points of minute heterogeneities in the reservoir. These fingers of displacing fluid tend to become extended in the direction of flow and travel at a faster rate than the remainder of the injected fluid, thus again resulting in premature breakthrough at the production system.
Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing fluid and thus avoid premature breakthrough. For example, it has been proposed in waterflooding operations to add thickening agents to at least a portion of the aqueous flooding medium in order to increase the viscosity thereof. The viscosity of the flooding medium may be increased prior to its injection into the reservoir or alternatively the viscosity may be increased in situ in order to avoid a reduction in injectivity at the injection well. For example, in U.S. Pat. No. 3,208,518 to Patton, there is disclosed a waterflooding process wherein the viscosity of the flooding medium is increased in situ through the use of high molecular weight polymers such as ionic polysaccharides produced by the fermentation of carbohydrates by bacteria of the genus Xanthomonas, under controlled pH conditions.
Various other techniques of improving sweep efficiency involve the selective formation of plugging agents within the more permeable zones of the reservoir. For example U.S. Pat. No. 3,522,844 to Abdo discloses a waterflooding process which employs a thickening agent which precipitates in the presence of divalent cations to form a plugging deposit in the reservoir. A chelating agent is injected in this process in order to control the formation of the plugging agent such that it is selectively precipitated at zones of crossflow between the strata of diverse permeabilities. In the Abdo process the thickening agent may be preceded by an aqueous solution of an alkali metal salt in order to effect an ion exchange reaction between the alkali metal ions and the divalent metal ions associated with the reservoir, thus placing the divalent metal ions in aqueous solution. Various other techniques for selectively plugging zones of an oil reservoir are known. For example, U.S. Pat. No. 3,396,790 to Eaton discloses a process wherein reactants are successively injected in a reservoir in order to produce an insoluble precipitate. U.S. Pat. No. 3,326,287 to Corrin and U.S. Pat. Nos. 3,476,187 and 3,618,664 to Harvey disclose the addition, to an aqueous driving fluid containing dissolved surfactants, of carbon black, kaolin and talc to function as carriers which transport the surfactants deeper into the stratum to more effectively displace oil.